This invention relates to a data backup method for a storage device, in particular, a method of storing differential snapshot data with efficiency.
Data in a storage device can be protected by several known protective functions, one of which is a snapshot function. The snapshot function is a function to keep data of a storage device at a designated time point (snapshot) and to provide the kept data to a user. A method to obtain the snapshot function is volume mirroring in which the entirety of a record area (primary volume) in a storage device is copied to another area (secondary volume) (refer to JP 2002-373093 A).
Another method proposed to obtain the snapshot function is differential snapshot in which update of data recorded in a primary volume is accompanied by copying of the updated data, instead of the whole data in the primary volume, to another volume (differential volume) for keeping. Since only updated data is copied, this differential snapshot method takes less disk capacity than volume mirroring which copies the entire volume. Still another method proposed is to make a system respond to a user faster when there is a data write request while a snapshot is accessed by the user (refer to JP 2003-280825 A). This method enables a user to acquire a large number of snapshots with ease.